ALIMEJ^TAEY CANAL OF VEETEBRATA. 555 



Alimentary Canal proper (Bnteron of the Trunk). 



§ 417. 



That portion of the tractus intestinalis^ which serves exclusively 

 for the ingestion and alteration of food, commences at the hinder 

 end of the cephalic enteron ; this, which is the digestive tube in the 

 strict sense, has an apparatus differentiated from it at its anterior 

 boundary, which forms an air-bladder in Fishes, where it is in an 

 indifferent condition, and a respiratory apparatus consisting of lungs 

 and trachea in the Amphibia and all higher forms. 



The most anterior portion of the digestive tube is not sharply 

 marked off from the cephalic enteron. As they are both innervated 

 by the vagus, there is reason for supposing that this portion was 

 primitively derived from the respiratory portion of the primitive 

 enteric tube after the atrophy of a large number of the hinder 

 branchial clefts, and that therefore it corresponds to the posterior 

 portion of the respiratory antechamber, which is so much larger in 

 Amphioxus. 



In the Craniota, not only sbme of the peculiar relations of the 

 rudimentary enteron, but also later stages in the development of this 

 tube, are due to the relations of the egg to the general rudiment of 

 the embryo, and to an increase in the quantity of the yolk. 



In the Selachii, the rudimentary enteron grows round the yolk, 

 but it is the groove-like portion only of the general rudiment, that 

 lying below the axial skeleton of the embryo, which is converted 

 into the enteron ; this is gradually shut off from the rest, or yolk- 

 beai'ing portion, which then appears as an appendage of the 

 enteron, the yolk-sac. This, which is at first placed apparently 

 outside the body, but which is surrounded by a continuation of the 

 integumentary layer, is merely connected by a stalk with the enteron 

 (external yolk-sac), and is gradually taken into the body (internal 

 yolk-sac). As the yolk is gradually used up, the yolk-sac is atrophied. 

 The Teleostei (and Ganoidei) are provided with a smaller quantity 

 of the nutrient material for the embryo, which constitutes the yolk. 

 Owing to the larger size of the yolk of the egg in Reptiles and Birds, 

 there is a similar contrast between the enteric canal and the yolk- 

 sac, but the latter is not covered by the integument, for the parts 

 which in the Anamnia enclose it go to form the amnion, and another 

 of the foetal coverings of the egg. In the Mammalia also, where the 

 material of the egg is very greatly reduced in quantity, the rudi- 

 mentary enteron becomes nipped off from the embryonic bladder, 

 which represents the yolk-sac (Fig. 319). This arrangement may be 

 deduced from a condition which was distinguished by the possession 

 of a large quantity of yolk-material. The want of a large quantity of 

 yolk in the Mammalia is compensated for by the development of 

 the foetus within the maternal organism, and the more or less close 



